A safety system for a fork lift truck is disclosed in U.S. Pat. No. 4,003,487 in the name of Terry R. Downing and having the same assignee as this invention which responds to excessive overturning moment acting on the truck by operating blocking hydraulic valves to closed positions wherein they disable the carriage elevating motor so that the carriage cannot be raised further and also disable the mast tilting motor to prevent further forward tilting of the mast. The hydraulic blocking valves have check valves which permit exhaust of fluid from the carriage elevating motor so that the carriage can be lowered and also permit supply of fluid to the mast tilting motor so that the mast can be tilted backward to remove an overload. However, a load may be picked up in such a manner as to create an excessive overturning moment and trip the protective system, but not allow relief of the overload by the normal operations of lowering the carriage or backward tilting of the mast permitted by the check valves. For example, this may happen when an overload picked up by first tilting the mast backward trips the protective system and there is insufficient clearance beneath the forks to allow relief of the load by lowering the carriage. Also when picking up a load from the floor by first tilting backward, the protective system may be tripped even if the weight is relatively small if the load is supported near the fork tips and acts together with the relatively long moment arm to create an excessive tilting moment. Under such circumstances when the load is picked up by backward tilting, the protective system cannot be reset by forward tilting of the mast which is inhibited by the tripped protective system. It is sometimes necessary with such safety system to utilize a second truck to relieve the overload before the protective system can be reset so that normal material handling operations are restored. A further disadvantage of such known overload protective system is that the check valves may become clogged and inoperative due to accumulation of dirt, thereby making it impossible to lower the load or tilt the mast backward to relieve the overload.